Exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as, but not limited to, carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions, typically disposed on catalyst supports or substrates, are provided in an engine exhaust system as part of an aftertreatment system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing CO and HC emissions is an oxidation catalyst (“OC”) device. The OC device includes a flow-through substrate and a catalyst compound applied to the substrate. One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device that may be positioned downstream of the OC device. The SCR device includes a flow-through substrate, having a SCR catalyst compound applied to the substrate.
During operation of the engine, the components of the after treatment system are exposed to sulfur from the fuel as well as the engine oil that are consumed by the engine. Over time as the sulfur accumulates, this affects the performance of the SCR catalyst as well as the oxidation catalyst. The sulfur is released from the catalysts of the OC device and the SCR device at relatively high temperatures (e.g., typically about 500° C. or greater), resulting in desulfurization.
Currently, an SCR efficiency diagnostic unit monitors the efficiency of the SCR device and indicates a failure of the SCR device if the efficiency of the SCR device falls below a threshold value. However, during normal operation of the SCR device high levels of sulfur in the SCR device may cause the SCR efficiency diagnostic unit to falsely indicate a failure of the SCR device. Accordingly, it is desirable to operate the SCR efficiency diagnostic unit to prevent false indications of a failure of the SCR device.